(1) Field of the Invention.
The invention relates to a system for the transmission of speech through a transmission path which is susceptible to interference, comprising a transmitter and a receiver which are coupled to the transmission path, the transmitter comprising means for converting the speech into an analog electric signal and the receiver comprising means for converting a derived analog electric signal into derived speech.
(2) Description of the Prior Art.
Mobile speech communication networks are confronted in certain circumstances by (possibly intentionally produced) serious interference in a (possibly varying) portion of the radio medium used, which seriously hampers the connections or renders them even completely unfit for use.
Methods are known for avoiding such interferences, the frequency of the radio channel used being changed in accordance with a predetermined schedule in such a way that all the available frequencies are cyclically passed through in a synchronous manner, for example in accordance with a (long) pseudo-random sequence (what is commonly referred to as frequency hopping).
A disadvantage is that generally the disturbed frequencies are also part of the "hopping schedule", so that the received speech signal continues to be disturbed its intelligibility is greatly reduced. The signal may also be disturbed due to, for example, a (temporary) loss of the hopping synchronism or because of the fact that a noise signal "follows" the hopping schedule.
A different method of trying to maintain the connection is, for example, the suppression at the receiving side of the noise signal ("null-steering") by means of a suitable antenna configuration and an associated signal processing system. However, the iterative process which must then be used requires additional time. The desired result is usually not attained because of an adverse signal-to-noise ratio or an interference coming from several directions.